With its high sensitivity, low amplification noise, high speed and high gain stability and uniformity from device to device, the phototransistor is a promising candidate for low light level sensing applications. One such device structure is disclosed in U.S. Pat. No. 7,067,853 entitled “Image Intensifier Using High-Sensitivity High-Resolution Photodetector Array”.
In a mesa semiconductor device structure the mesa surfaces, including top surface and sidewall surfaces, are often passivated with a passivation material to increase device performance and reliability. A number of different dielectric materials have been tried as passivation materials. These passivation materials have been fabricated using a wide range of deposition techniques and thin film growth techniques. Only the specific combination of high dielectric constant (high-k) dielectric material deposited using atomic layer deposition (ALD), as disclosed in U.S. patent application Ser. No. 12/244,938, has had some success in the passivation of heterojunction phototransistor (HPT) mesa sidewalls. This technique, however, now requires extensive fabrication process development and may be relatively costly to manufacture. A passivation method utilizing existing fabrication facilities, equipment, and processes in the semiconductor photodetector industry may be desirable.